This invention relates to electrodeposition coating systems using liquid coating materials contained in tanks wherein the coating liquid is electrically charged and workpieces which are moved through the tanks for coating are oppositely charged. More particularly, the invention is an improved electrodeposition coating apparatus which provides an enhanced and improved electrically deposited coating on the workpieces through improved circulation of the liquid coating material within the holding tank and improved electrical charging capabilities coupled with reduced electrical power requirements.
In the automobile, metal office furniture and other industries using metal parts, advancements in the painting of metal parts have occurred in recent years. A well accepted painting method in such industries is that of electrodeposition of coating materials on metallic parts, which are usually charged positively while the liquid coating material is negatively charged. As a result, the coating material is attracted to and strongly adhered to the metal part. Especially in automobile parts, which are typically exposed to extreme weather conditions and corrosive atmospheres, electrodeposition painting or coating provides significantly better corrosion resistance, namely, at least three to five times better than parts painted with previously known methods.
In many conventional electrodeposition coating systems, water borne paint pigment such as a resin is kept in solution by circulation of the liquid within the painting tank. The water based coating material must conduct electricity therethrough to the items to be painted which are supported within the tank. Accordingly, numerous structures for accomplishing these purposes have been used and proposed including U.S. Pat. Nos. 2,710,832 to Harr, 3,496,082 to Orem et al. and 3,592,755 to Thornton which exemplify conventional apparatus for mounting electrodes within electrolytic deposition tanks and circulating the coating liquid within the tank.
Circulation of the liquid coating material within the electrolytic deposition tank has been a significant problem in many prior structures, however. Electrodes required for charging the liquid coating material have been positioned within the tank and connected to piping also contained within the tank in such a way that flow through the tank for circulation of the liquid and maintenance of the paint solution has been severely disrupted. Also required within the tank were rails and other protective structure to guard the electrodes from damage. This also disrupted circulation. The mounting of the electrodes, piping and protective structure within the tank has also restricted the work area available for painting within the tank. These combined problems have severely affected painting quality.
Additional problems with previously known paint systems include relatively high electrical power requirements needed to charge the coating material for proper paint adherence to workpieces being painted. The prior known tank structures required significant distances between the workpieces and the electrodes because of the requirements for positioning the electrodes and circulation equipment within the tank. Maintaining the electrical charge over such distances required relatively high electrical "throw-power". Moreover, many of the tanks which utilized liquid coating material circulation apparatus provided inadequate protection for the circulation equipment within the tank. When heavy parts being painted fell from supporting racks within the tank, the circulation equipment was often damaged by the falling or sinking objects. Thus, circulation within the tank was further disrupted.
Yet another problem was the contamination of the liquid coating material within the tank during use. Many prior known electrodeposition coating systems do not include covers over the tank area. This allowed the entry of dirt, foreign objects and other contaminants into the tank causing problems in maintaining the coating material in a proper state for good adherence and coating quality. Further, since the coating material was charged with electricity and thereby increased in temperature during operation of the coating apparatus, those tanks which were covered often encountered condensation problems on the covers causing dripping on the painted articles and reducing overall paint quality.
The present invention was conceived and made in recognition of the above and other problems in electrodeposition coating and provides significant advantages over prior systems.